The first objective of this research is to continue investigations of the control mechanisms that regulate proliferation and differentiation of hemopoietic stem cells derived from normal bone marrow. We propose to isolate and study colonies of hemopoietic cells as they repopulate the spleen of an irradiated mouse. Using autoradiographic and light and electron microscopic techniques, we wish to examine the number and distribution of replicating or differentiating stem cells within central and peripheral regions of a colony. After mapping zones of cellular replication or specialization, we will seek to clarify mechanisms of cell-cell communication governing those processes. The second objective is to provide information about the recovery of hemopoiesis in mice with tumor that would permit the design of anticancer regimens to enhance the differential sensitivity of normal and neoplastic cells to irradiation. Not only will results from these studies enhance our understanding of the proliferation and differentiation of normal tissues, and by comparison abnormalities associated with neoplastic growth, but it should provide an insight into hemopoietic cell recovery versus tumor cell growth to develop new therapy and improve current regimens.